Diesters of naphthalene dicarboxylic acid find general and useful application in the preparation of high performance polymeric materials, such as polyesters and polyamides. Of the diesters, dimethyl-2,6-naphthalene dicarboxylate (hereinafter, referred to as “NDC”) is taken as representative. Poly(ethylene-2,6-naphthalene) (hereinafter, referred to as “PEN”), one such high performance polyester, is prepared by the condensation of NDC with ethylene glycol. Fibers and films made from PEN are found to have advantages in terms of strength and thermal properties over poly(ethylene terephthalate) (hereinafter, referred to as “PET”). Thanks to its excellent physical properties, PEN is used to form thin films useful in the preparation of magnetic recording tapes and electromagnetic parts. In addition, because of its superior resistance to gas diffusion, and particularly to the diffusion of carbon dioxide, oxygen and water vapor, films made from PEN are applicable for manufacturing food containers, particularly “hot fill” type food containers. Also, PEN can be used to make high strength fibers useful in the preparation of tire cord.
Nowadays, NDC, as shown in FIG. 1, is generally produced by oxidizing DMN into crude naphthalene dicarboxylic acid (cNDA), followed by esterification. In most current cases, NDC is used as a main material for the synthesis of PEN. However, NDC suffers from several problems, compared to NDA. First, NDC is condensed into PEN with the concomitant production of methanol, which carries a danger of explosion, while water is produced upon the condensation of NDA. Next, because it is obtained from NDA by esterification and purification, NDC requires one more process compared to NDA. Also, NDC cannot take advantage of PET production facilities that may already exist. Despite such disadvantages, NDC is usually used, instead of NDA, for the synthesis of PEN because NDA has not been prepared with sufficient purity for use in the synthesis of PET.
The oxidation of DMN leads to cNDA, with by-products, such as 2-formyl-6-naphthoic acid, 2-naphthoic acid, etc., concomitantly produced as a result of incomplete oxidation, as seen in FIG. 2. The impurities, particularly, FNA, if present, cause breaks during the polymerization of PEN into polymeric materials, thereby having a bad influence on the properties of the polymers. To apply cNDA for the synthesis of PEN, FNA must be removed therefrom in advance, but it is difficult.
There are various methods known to remove FNA from cNDA reactions. For example, recrystallization for purifying NDA, a repetition of oxidation, and conversion of cNDA into NDC in the presence of methanol, followed by either hydration or hydrogenation into purified NDA have been proposed. Additionally, attempts have been made to remove FNA by solvent washing, melt crystallization, high pressure crystallization, supercritical extraction, etc. However, these techniques just account for NDA with insufficient purity. On the other hand, conventional methods can increase the purity of NDA, except making a sacrifice of yield, so that they are difficult to apply in practice.
As described above, chemical or physical methods for NDA production are given problems, including the production of environmental pollution, the likelihood of explosions due to high temperatures and pressures, an increase in production costs due to large-scale facilities, large energy consumption, etc. For direct use in polymerization to high performance polymeric material, 2,6-naphthalene dicarboxylic acid must be of high purity, which is difficult to attain with conventional methods. An additional purification process, even if capable of achieving the high purity in NDA, gives rise to productivity reduction and process extension.
Thus, extensive attention has recently been paid to biological methods, especially using microorganisms. Previously, the present inventors developed FNA removal using a novel Bacillus sp. as disclosed in Korean Pat. Appl'n No. 2002-0087819 and proposed a method for preparing aromatic aldehydes and carboxylic acids in the presence of xylene monooxygenase as disclosed in Korean Pat. Appl'n No. 2002-7005344. Nowhere is the conversion of FNA into NDA using benzaldehyde dehydrogenase derived from Sphingomonas aromaticivorans described in the previous literature.